[Heath] SB-200 problems...again...

[Bill Bear]

Ken,

I’m embarrassed to tell you how I solved the “no idle plate current” problem.  Nothing wrong with the SB200.  All mods wired correctly and nothing else wrong.  It was the “operator”…..  I was trying to read idle plate current with the meter switch in the “grid” position.  Made all the difference in the world when I realized my mistake and switched to the “plate” position… 90 mils as advertised !

But I’m sure I have not been the only one to do that.

73

Bill, KL7YB


On May 6, 2016, at 8:21, Kenneth G. Gordon <kgordon2006 at frontier.com> wrote:

> On 6 May 2016 at 7:46, Bill Bear wrote:
> 
>> Ken,
>> 
>> One of the first things I did after an initial visual inspection was replace C19
>> ( 2uF) and C3 (20 uF).  They just looked too old and my experience with old
>> electrolytics is that they are no longer up to specs.  So, I must have something
>> else to investigate.
> 
> OK. Let me see if I can remember exactly what I ran into: for one thing, I 
> began having that problem after I had installed a Harbach Soft-Key interface 
> to help my more modern transceivers key the amp. 
> 
> It turned out that the high 60 Hz AC ripple in the DC to the grids was causing 
> the Soft-Key to partially key on, increasing the bias voltage to below ("above" 
> i.e. more negative)  -2 volts.
> 
> In order for the 572Bs to draw 90 mA of idle current, the grid bias must be  
> -2VDC. If the voltage increases "above" that (becomes more negative), the 
> idle current will drop, and the tubes will possibly be operating nearer a 
> non-linear region of their operating curve.
> 
> Secondly, there is also the fact that in order to read that 90 mA, the metering 
> circuits have to be working properly. That 90 mA shows at the very bottom of 
> the scale and in fact is very hard to read there, and is not very accurate. 
> 
> What one really should do is to add another section or rewire a seldom-used 
> section, of the meter switch to read idle current at a point at least half-scale 
> on the meter.
> 
> Thirdly, idle current will also fall if plate voltage is not close to where it should 
> be. I get 2150 VDC under load.
> 
>> After you changed C19, did you get a nominal idle plate current reading near 90
>> ma?
> 
> Yes.
> 
>> I am also wondering what resistance check results you had after the mods: 
>> plate to gnd and tube pin 3 to gnd ?
> 
> I have not checked those, but will as soon as I can. 
> 
> My most-used SB-200 has recently come up with a problem very similar to 
> yours in that both idle current has pretty much disappeared, and it is requiring 
> a lot more drive to get the grid current and power output to be where it should 
> be. 
> 
> Tubes are brand-new RF Parts jobs too. And this problem suddenly occurred. 
> This indicates to me that some component in the grid circuit has suddenly 
> failed.
> 
> Due to our youngest daughter's upcoming wedding, I have not been able to 
> do much radio operating for the past several months, but hope to be able to 
> get back to that after May 21. :-)
> 
> Therefore, I have not been able to dig into the SB-200
> 
> vy 73 for now,
> 
> Ken W7EKB
>